Switching regulators are used for converting an unregulated power source to a regulated voltage and/or current source. FIG. 1 shows a traditional switching regulator with synchronous rectification. A first switching signal S1 is utilized to control the duty cycle of a first switch 10 for the regulation of voltage and/or current. As shown in FIG. 2, a current IIN is to charge an output capacitor 40 during the on time of the switch 10. FIG. 3 shows a second switching signal S2 for turning on a second switch 20 in response to the turning off of the first switch 10 for providing a low impedance path for a discharge current IF of an inductor 30. In continuous current mode (CCM) operation, the first switch 10 is turned on before the energy of the inductor 30 is completely discharged. In the discontinuous current mode (DCM) operation, the energy in the inductor 30 is fully discharged before the start of the next switching cycle. FIG. 4 shows a reverse current IR discharging the output capacitor 40 through the second switch 20 during the DCM operation. The reverse current IR will cause power losses and the lowering the switching regulator efficiency at light load and no load conditions. FIGS. 5A and 5B show a plurality of CCM and DCM waveforms, respectively.
Conventional methods for limiting the reverse current in a synchronous rectification circuit include the use of a current sensing circuit for turning off the synchronous rectifier once a reverse current is detected. The current sensing circuit involves the use of the turn-on resistor (RDS-ON) of the transistor (synchronous rectifier) or a series resistor for detecting the reverse current. However, these approaches cause power losses and add complexity to the system. Moreover, the synchronous rectifier can only be turned off after the reverse current is generated and detected. Accordingly, a synchronous rectification circuit that eliminates the effects of reverse current without the current sensing circuit would be advantageous.